In mobile phones, tablet PCs, and other portable USB devices, a power adapter or a mobile power supply adopted should use an appropriate mechanism for a Universal Serial Bus (USB) to identify different types of devices, so as to output different currents for different types of devices; for example, a level value or a communication state of a USB communication line D+/D− can be set to achieve the recognition of different devices.
Existing power amplifiers are usually built with a battery, which has the important condition to achieve mobile power function. However, when power is supplied to a USB device by using a power amplifier, improper charging current provided by the power amplifier is easy to lead to problems. For example, the current output capability of the mobile power supply cannot be sufficiently exerted if the current used for charging is small; however, output overload protection may occur if the current used for charging is large, this may lead to a failure in charging or the actual current used for charging is too small.